<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    *{
      margin: 0;
      padding: 0;
    }

    body{
      background: url(images/1.jpg) no-repeat center top;
    }

    li{
      list-style: none;
    }

    .baidu{
        overflow:hidden;
        margin: 100px auto;
        background-color: #fff;
        width: 410px;
        padding-top: 3px;
    }
    .baidu li{
      float:left;
      margin: 0 1px;
      cursor:pointer;
    }
    .baidu img{
      width: 100px;
    }
  </style>
</head>
<body>
  <ul class="baidu"> 
    <li><img src="../images/1.jpg" ></li>
    <li><img src="../images/2.jpg" ></li>
    <li><img src="../images/3.jpg" ></li>
    <li><img src="../images/4.jpg" ></li>
  </ul>
  <script>
    //1.获取元素
    var imgs = document.querySelector('.baidu').querySelectorAll('img');
    console.log(imgs);
    //2.循环注册事件
    for(var i =0; i<imgs.length; i++){
      imgs[i].onclick = function(){
       console.log(this.src);
       //把这个路径 this.src 给body 就可以了
       document.body.style.backgroundImage = 'url('+ this.src +')'
      }
    }
  </script>
</body>
</html>